We all have different situations, experience, needs, expectations and more when it comes to starting stuff from seed, and of course gardening in general. I can't express the appreciation for comments and suggestions from nubes to seasoned on this.
I am entering my 2nd year of selling fresh cut herbs at the farmers market. Most all are grown in containers, from 10" to big. The 10" mainly for ones that may have to be wintered inside or rolled in and out. So come warm weather few of the smaller pots. I've messed with gardening since forever, large and small. I also worked at garden centers (until 2 yrs ago) and never thought about starting from seed. We had pretty starts coming in all the time!
As the page turns, I am starting a few more from seed. No greenhouse, no big system, just doing what works for me. I use the peat pellets, 10 -12 in a small tray. I can get 3 -4 on top of frig. (there's other stuff up there including the radio) Once sprouted and needing light get shifted to another area including window sills and shelves with window and additional lights.
Once these infants are up and have real leaves usually go to a 4" pot. The dirt is mostly potting soil, using Fafards now, a bit of worm castings, and some bagged top soil. The netting on the pellet gently pulled back, some cut off, and usually 2 opposite corners put in pot. When it comes time to put in garden or regular container the two separate easily.
A couple of plants are direct sown into pots, and this includes nasturtium and dill. Nasturtium is a fleeting one for us, passing out in June with heat. I have to work on the dill.
The dirt make up in the larger outdoor pots is a bit different, and that is another topic!
I had some need to write this out, knowing we all have different situations.